This application relates to the finishing of polycarbonate using a melt process, and in particular to a method for quenching residual catalyst used in the polycarbonate-forming reaction, and the products formed by this reaction.
Aromatic polycarbonates are useful in a great many applications because of their desirable physical properties, including strength and optical clarity. There are three processes known for the production of aromatic polycarbonates, which are illustrated in FIG. 1. The conventional interfacial process and the phosgene-based melt process start with the reaction of phosgene with carbon monoxide. The "no phosgene" melt process was developed to eliminate the use of highly toxic phosgene in the reaction process.
Both types of melt processes make use of a diarylcarbonate such as diphenylcarbonate (DPC) as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A (BPA) in the presence of an alkaline catalyst to form a polycarbonate in accordance with the general reaction shown in FIG. 2. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers. In many cases, however, the presence of residual catalyst has a detrimental effect on the quality of the product, leading to poor color, molecular weight or rheological properties. Residual catalyst may also interact with additives, detracting from their efficacy. Thus, it is desirable to reduce the levels of residual catalyst to minimize these interactions. Such reduction is referred to as "quenching."
Commonly assigned U.S. Pat. No. 5,606,007, which is incorporated herein by reference, discloses the use of acidic compounds to quench residual alkalinity. The acid compounds tested are shown to produce polycarbonates with improved heat and water resistance, and low yellowness indices. An important consequence of residual alkaline catalyst which is not directly addressed by this patent is the base-catalyzed coupling of UV absorbers to the polycarbonate backbone. Normally, amounts of liquid quencher of less than 4 ppm are utilized. Although this amount is small, it corresponds to approximately 4 times the theoretical amount of quencher which should be necessary to neutralize all of the catalyst used. Nevertheless, despite the excess of quencher, reaction between UV absorber and polycarbonate backbone still occurs, impairing the efficiency of UV protection. Furthermore, it does not appear that the addition of more quencher has any significant effect on the levels of reaction between the UV absorber and the polycarbonate once a threshold has been reached. Thus, there remains a need for a method for quenching residual alkaline catalyst which more effectively reduces the interaction of UV absorbers with polycarbonate.
It would be desirable to provide such a method.
It would further be desirable to provide finished polycarbonate compositions with high UV retention values and good color stability.